<p>Alien invasive species impact native plant communities, not only through direct negative effects on native species but also by altering nutrient cycling and availability. However, the mechanisms driving these changes—such as differences in litter decomposition rates and litter quality, or increased overall decomposition rates within invaded stands—remain unclear. This study examines how <i>Solidago gigantea</i>, an invasive species in Europe and Asia, affects decomposition. A climate chamber experiment tested whether <i>S. gigantea</i> litter decomposes faster than litter from co-occurring native species. The study examined whether differences in decomposition rates could be attributed to litter quality (using C/N ratio as a proxy) or to variations in soil microbiota (via inoculum from invaded vs. non-invaded plots). A field experiment measured overall decomposition rates in invaded and non-invaded plots using tea bags and wooden spatulas buried for 2–16 weeks. Soil moisture, carbon, and soil fauna activity were also assessed for their influence on decomposition. <i>S. gigantea</i> litter decomposed significantly faster than native graminoid species (decomposition rates of 0.91 and 0.33 g g⁻¹ day⁻¹ resp.), despite its higher C/N ratio (39.6 and 27.9 resp.). Invaded stands consistently had higher decomposition rates, which was attributed to abiotic changes, including reduced soil moisture and increased soil carbon, rather than to biotic changes in soil fauna or microbiota activity. These findings highlight <i>S. gigantea</i>’s substantial impact on nutrient cycling in invaded ecosystems. However, the extent—and potentially the direction—of these effects likely depends on the invaded plant community. Grass-dominated and wetter communities may experience the greatest increases in nutrient cycling, potentially enhancing primary productivity.</p>